In motor vehicles with internal combustion engines, transmissions are usually used in order to convert the speeds and torques provided by the internal combustion engine, as a function of the driving range at the time, into speeds and torques suitable for the propulsion of the motor vehicle. For reasons relating to driving stability and traction these converted speeds and torques must thereby be transmitted to at least one axle of the motor vehicle, i.e. the power available must be distributed at least to a driven wheel on the left and one on the right. However, the use of a common driveshaft for both driven wheels would give rise to the problem that when driving round curves the drive-train would be stressed and the wheels would be subjected to more severe wear and higher loading, since the wheel on the outside of the curve has to cover a larger distance than the wheel on the inside. For that reason a so-termed differential gearbox is usually provided between the motor vehicle transmission and the axle shafts of the driven wheels, which enables unconstrained speed and force compensation between the axle shafts of the driven wheels. For this purpose the power coming from the transmission of the motor vehicle is transmitted to a housing of the differential gearbox and, with the help of compensating elements, divided by it between two output gears arranged inside the housing, each connected to a respective axle shaft. When the motor vehicle is driving straight ahead, the compensating elements rotate together with the housing of the differential gearbox as a block and distribute the power supplied by the motor vehicle transmission uniformly to the two output gears and hence to the axle shafts. In contrast, when the motor vehicle is driving round a curve the compensating elements inside the housing undergo compensatory movement in order to enable the two output shafts to run at different speeds.
However, when the friction-grip potentials of the two driven wheels are different, the action of the differential gearbox has the decisive disadvantage that the propulsion forces that can be transmitted to the road depend on the lower of the friction-grip potentials. In an extreme situation the result is that if one driven wheel is spinning, no torque higher than that transmitted by the spinning wheel can be transmitted by the respective other wheel, so the motor vehicle cannot be driven at all, or only to a very limited extent. To overcome this drawback it is usual to provide blocking devices in the area of the differential gearbox, in order to suppress completely or at least impede the compensatory movement of the compensating elements.
From DE 38 29 701 A1 a differential gearbox is known in which, by means of a housing, output gears at least within the housing can be driven by compensating elements, so that different speeds of the output gears can be produced by corresponding compensatory movements of the compensating elements. To be able to selectively bridge across the action of the differential gearbox in certain situations, a friction clutch is provided between one of the output gears and the housing, which when actuated couples this output gear directly to the housing and in this way ensures that the rotational speed of the housing and the output gear are equal. A firm coupling of the output gear to the housing, by virtue of the compensating elements, brings about an entrainment of the respective other output gear at the same rotational speed and consequently forces a uniform division of the power between the two wheels. To actuate the friction clutch, an actuating chamber is provided in its area, which is delimited by a static and a working part and is filled with a hydraulic fluid. When pressure is built up in this actuating chamber the working part is moved toward the friction clutch and transmits this movement, via interposed elements, to one of the two sides of the clutch, so causing it to close. When the pressure in the actuating chamber is reduced, the clutch is opened by virtue of a return movement of the working part brought about by spring elements arranged in that area.
It is generally known that especially in front-wheel-drive motor vehicles, a differential gearbox of this type can be accommodated in a main chamber of the motor vehicle transmission, so that in this case the housing of the differential gearbox is driven by a main shaft. The rotation of the main shaft is produced by the converted transmission of rotation of an input shaft also arranged in the main chamber.
However, such a design of a motor vehicle transmission has the disadvantage that because of leakage the hydraulic fluid provided for actuating the friction clutch can make its way into the main chamber of the motor vehicle transmission, and can mix with the transmission oil present there, which is different from it. Particularly for the shifting processes taking place between the driveshaft and the main shaft, this transmission oil has tailor-made properties which are affected progressively adversely as more and more of the hydraulic fluid from the actuating chamber mixes with it. As a result, over its lifetime the shift quality of the motor vehicle transmission also deteriorates progressively.